This invention relates to adjacent channel power of signal sources. In particular it relates to a method and apparatus for measuring the adjacent channel power of, for instance, a transmitter.
It is often required to take measurements of the characteristics of a transmitter to ensure that the transmitter does not cause interference to users of adjacent channels in the radio spectrum. Measurements are taken of the amount of signal power, either in absolute terms or relative to the whole transmitted power, that a transmitter emits in the adjacent channels to the transmission channel. Adjacent channel power is usually measured over a bandwidth equivalent to that o a typical receiver, which varies upon the type of transmission i.e, single sideband (SSB), frequency modulation (FM), etc. In a typical SSB transmission the adjacent channel is nominally 2.4 kHz wide, separated by 5 kHz from the main transmission channel.
Conventionally, such measurements are made by firstly converting the transmitter signal to a suitable intermediate frequency, i.f. The signal is subsequently passed through a bandpass filter centered on the adjacent channel to be measured, and having well defined characteristics. For example, for an SSB transmitter the bandpass filter may have typically a 6 dB bandwidth of 2,400 Hz. Steep skirts are required to substantially reject signals outside the required bandwith, including of course the main transmission signal. The filter must therefore be different for each type of transmission, must be environmentally stable, and requires a very high Q. They are usually ceramic, crystal or mechanical filters. As a result they can be expensive to produce and test and a different design is required for each transmission system.
According to the present invention, there is provided a method for measuring adjacent channel power of a signal source including down converting a signal such that the adjacent channel band is converted to base band; filtering the down converted signal with a low pass filter; and measuring the power output from the low pass filter.
In practice as will be further described, the meters used to measure the power output must be a.c. coupled in order not to measure d.c. offsets from the filter and amplifiers. As is also described below this leads to an unwanted notch in the filter characteristics. To overcome this problem, the invention also provides a method for measuring adjacent channel power of a signal source including the steps of mixing an input signal with a local oscillator signal having angle modulation and which is nominally centered on the desired adjacent channel centre frequency so that the adjacent channel band is down converted to base band: filtering the down converted signal with a low pass filter; and measuring the power output from the low pass filter by means of an a.c. coupled meter.
According to a first aspect of the invention, the local oscillator is modulated by phase or frequency modulation which may be from a noise source, and the low pass filter has a frequency response substantially equal to half the adjacent channel bandwidth.
Typically the noise may be restricted in bandwidth and frequency and may for example have a 50 Hz bandwidth and a r.m.s deviation of 50 Hz.
The output signal from the low pass filter may be amplified and measured by a volt meter and a value obtained for the power, either absolute or relative, of the adjacent channel.
According to a second aspect of the invention the low pass filter may have a bandwidth significantly less than the adjacent channel bandwidth. Typically this might be 100 Hz. In this case the local oscillator frequency may be swept by a periodic waveform, such that parts of the desired adjacent channel frequency spectrum are passed in turn by the low pass filter. The periodic waveform may be a triangular wave. The output from the low pass filter may be arithmetically squared and measured by an integrating type volt meter adapted to measure the output voltage over one or more complete cycles of the local oscillator sweep waveform. The output from the volt meter is then a measure of the adjacent channel power.